Re: [PATCH v4] bpf: core: fix shift-out-of-bounds in ___bpf_prog_run

[Yonghong Song]

On 6/5/21 12:10 PM, Alexei Starovoitov wrote:
> On Sat, Jun 5, 2021 at 10:55 AM Yonghong Song <yhs@xxxxxx> wrote:
> > On 6/5/21 8:01 AM, Kurt Manucredo wrote:
> > > Syzbot detects a shift-out-of-bounds in ___bpf_prog_run()
> > > kernel/bpf/core.c:1414:2.
> >
> > This is not enough. We need more information on why this happens
> > so we can judge whether the patch indeed fixed the issue.
> >
> > > I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid
> > > missing them and return with error when detected.
> > >
> > > Reported-and-tested-by: syzbot+bed360704c521841c85d@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > Signed-off-by: Kurt Manucredo <fuzzybritches0@xxxxxxxxx>
> > > ---
> > >
> > > https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231
> > >
> > > Changelog:
> > > ----------
> > > v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals.
> > >        Fix commit message.
> > > v3 - Make it clearer what the fix is for.
> > > v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary
> > >        check in check_alu_op() in verifier.c.
> > > v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary
> > >        check in ___bpf_prog_run().
> > >
> > > thanks
> > >
> > > kind regards
> > >
> > > Kurt
> > >
> > >    kernel/bpf/verifier.c | 30 +++++++++---------------------
> > >    1 file changed, 9 insertions(+), 21 deletions(-)
> > >
> > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > > index 94ba5163d4c5..ed0eecf20de5 100644
> > > --- a/kernel/bpf/verifier.c
> > > +++ b/kernel/bpf/verifier.c
> > > @@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
> > >        u32_min_val = src_reg.u32_min_value;
> > >        u32_max_val = src_reg.u32_max_value;
> > >
> > > +     if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) &&
> > > +                     umax_val >= insn_bitness) {
> > > +             /* Shifts greater than 31 or 63 are undefined.
> > > +              * This includes shifts by a negative number.
> > > +              */
> > > +             verbose(env, "invalid shift %lld\n", umax_val);
> > > +             return -EINVAL;
> > > +     }
> >
> > I think your fix is good. I would like to move after
>
> I suspect such change will break valid programs that do shift by register.

Oh yes, you are correct. We should guard it with src_known.
But this should be extremely rare with explicit shifting amount being
greater than 31/64 and if it is the case, the compiler will has a
warning.

> > the following code though:
> >
> >           if (!src_known &&
> >               opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND) {
> >                   __mark_reg_unknown(env, dst_reg);
> >                   return 0;
> >           }
> >
> > > +
> > >        if (alu32) {
> > >                src_known = tnum_subreg_is_const(src_reg.var_off);
> > >                if ((src_known &&
> > > @@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
> > >                scalar_min_max_xor(dst_reg, &src_reg);
> > >                break;
> > >        case BPF_LSH:
> > > -             if (umax_val >= insn_bitness) {
> > > -                     /* Shifts greater than 31 or 63 are undefined.
> > > -                      * This includes shifts by a negative number.
> > > -                      */
> > > -                     mark_reg_unknown(env, regs, insn->dst_reg);
> > > -                     break;
> > > -             }
> >
> > I think this is what happens. For the above case, we simply
> > marks the dst reg as unknown and didn't fail verification.
> > So later on at runtime, the shift optimization will have wrong
> > shift value (> 31/64). Please correct me if this is not right
> > analysis. As I mentioned in the early please write detailed
> > analysis in commit log.
>
> The large shift is not wrong. It's just undefined.
> syzbot has to ignore such cases.

Agree. This makes sense.